convert mp3 to m4b on the linux command line

I quite often want to convert mp3 files to m4b, Apple’s proprietary nastiness. There is pacpl of course but that has issues these days, particularly with copying the id3 tags to the new file.

FFmpeg can do it when you build from source (I just found a nice script that will build it from source for you). So I came up with this way to do it from the command line. It uses an awesome feature of xargs which will run it in parallel, so one for each core.